The goal of this project is to understand how murine leukemia viruses (MuLV) cause lymphocytic leukemia. We have sought to 1) identify preleukemic changes in the lymphoid cells of MuLV-infected animals using fluorescent labeled antibody to viral and lymphoid differentiation antigens and the fluorescence activated cell sorter, and 2) determine the region of the MuLV genome responsible for leukemogenesis of specific lymphoid cells by generating recombinants between a virus that causes T cell leukemia and a virus that causes B cell leukemia.